create network cloned devices without specifying a unit

[Darren Reed <darrenr%netbsd.org@localhost>]

A diff has been attached to the following PR:

kern/46633: create network cloned devices without specifying a unit

It should eventually show up here:
http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=46633
(but is not there currently)

Testing:

firewall# ./ifconfig tun create
tun0
firewall# ./ifconfig tun create
tun1
firewall# ./ifconfig gif create
gif0
firewall# ifconfig tun1
tun1: flags=10<POINTOPOINT> mtu 1500
firewall# ifconfig gif0
gif0: flags=8010<POINTOPOINT,MULTICAST>
firewall# ifconfig gif0 destroy
firewall# ifconfig tun1 destroy
firewall# ifconfig tun0 destroy
firewall# ifconfig tun0
ifconfig: SIOCGIFFLAGS tun0: Device not configured

Comments?
Criticisims?
Code review?

Darren